Sheathed-element glow plugs are usually used to heat up the combustion chambers when an internal combustion engine is started.
A sheathed-element glow plug is an electric heating element in the combustion chamber of an internal combustion engine. The sheathed-element glow plug is heated electrically only briefly at the start. The diesel fuel injected into the combustion chamber during a cold start of a diesel engine usually does not spontaneously ignite as smoothly as described in the theory of the diesel process.
For these reasons, an electrically heatable sheathed-element glow plug is inserted into the combustion chamber and is preheated in the startup phase. This is also known as preheating. The current required for this equals approximately 20 to 40 amperes per cylinder.
However, the diesel fuel injected into the combustion chamber during a cold start of a diesel engine does not usually spontaneously ignite as smoothly as described in the theory of the diesel process.
The reasons for this include the fact that the walls of the combustion chamber (cylinder walls, piston bottom) are still cold and have a high specific thermal capacity (iron material) while compressed air has a low thermal capacity. Therefore, the heat of compression is rapidly transferred to the cylinder walls and the piston base.
Another reason for this is that during startup, the piston speed is lower due to the electric starter motor (starter) and therefore there is more time for transfer of heat from the compressed air to the wall. Chamber engines in particular have a larger effective surface area, which absorbs heat from the gas. Starting a cold engine without sheathed-element glow plugs is possible above air temperatures of −10° C. in the case of direct injection, +30° C. in swirl chamber injection and approximately +60° C. in prechamber injection.
It is a disadvantage that when the engine is cold, compressed air may escape out of the combustion chamber past the piston rings, so that the final compression pressure and thus the final compression temperature turn out lower. These losses are further increased due to the lower piston speed during startup.
Another cause of the reduced combustion quality may be due to different fuel grades, in particular when the engine is flex-fuel-capable and is to burn fuels that are not easily ignitable.
FIG. 3 shows the structure of a closed-loop T regulation in combination with an open-loop T control, such as that known from the related art. An engine control EDC (electronic diesel control) is connected to a glow control unit GCU via an interface. A closed loop contained in the glow control unit GCU includes a temperature regulator TR, a resistance regulator RR, a sheathed-element glow plug output GP and a plug temperature model device TM, which are interconnected in this order. A first connecting loop P1 (path 1) is provided between the output of plug temperature model device TM and the input of temperature regulator TR.
A first subtraction circuit SI is provided at a connection point at the input of temperature regulator TR, temperature signals from both connections being input into this circuit More specifically, a temperature setpoint value Tsetpoint that is supplied to glow control unit GCU is applied to first subtraction circuit S1 and an actual temperature value Tactual supplied by first connection loop P1 is also applied there. First subtraction circuit S1 calculates a temperature difference value ΔT from these two values and sends this temperature difference value ΔT to the input of temperature regulator TR.
Temperature regulator TR calculates a setpoint resistance value Rsetpoint and sends this setpoint resistance value Rsetpoint to a second subtraction circuit S2 connected between temperature regulator TR and resistance regulator RR. An actual resistance value Ractual is sent via a second connecting loop P2 (path 2) to another input of second subtraction circuit S2. This actual resistance value Ractual is calculated from the quotient of an effective voltage value Ueff available at the output of resistance regulator RR and a measured current value Imeasure available at the output of sheathed-element glow plug GP.
Second subtraction circuit S2 calculates the difference between setpoint resistance value Rsetpoint and actual resistance value Ractual and outputs a differential resistance value ΔR at the output. This differential resistance value ΔR is sent to resistance regulator RR. Actual temperature value Tactual is available at the output of this closed control loop. It is inherent in this actual temperature value Tactual that the modeled temperature of sheathed-element glow plug GP is not measured.